Here is some info. hope it helps a little..
HES (hydroxy-ethyl starch, a chemically modified form of corn starch), they could achieve performance enhancing combination of elevated total blood volume and high hemoglobin, all while holding hemoglobin below the legal limit. But, HES was officially placed on the banned substances list 1 year ago as a masking drug.
Probenecid---The Masking Drug for steroids
The action of probenecid is to slow down both penicillin and it appears also other prohibited substances, such as anabolic steroids, from being excreted into the urine. Hence, it is a prohibited drug because of its retention capacity would work to lower the amount of excreted substance. Having that effect probenecid is categorized as a masking agent, but in the absence of any other banned drug it would not enhance performance. Also, there is no evidence to show that probenecid prevents steroids or any banned substance being detected with the new, very sensitive, high-resolution mass spectrometers used for drug testing. Any Olympic accredited drug laboratory will agree with this. The only basis for this being a banned substance is suspicion and that is not grounds for punishment.
A large dose might be taken as soon as an athlete is aware of unannounced testing, such as when testers arrive at a person's home or the athlete is forewarned.
The athlete would have to urinate, but not as a sample, to remove existing traces of performance-enhancing substances.
The athlete would then delay until the bladder filled enough to provide a urine sample, allowing the large amount of Probenecid to block excretion of the performance-enhancing substance.
The urine sample would be clear of performance-enhancing substances but would be exceptionally high in Probenecid content.